1. Field of the Invention
This invention relates to a carrier for electrophotography. This invention also relates to a two-component type developer for developing electrostatic images that comprises the carrier and a toner.
2. Related Background Art
In general, in electrostatic recording systems making use of electrophotography, commonly employed is a method in which a photoconductive material such as selenium, OPC (organic photoconductive material) or .alpha.-Si is used in a photosensitive member, where the photosensitive member is uniformly charged by various means, thereafter the charged surface of the photosensitive member is irradiated with a light image to form on its surface an electrostatic latent image corresponding to the light image, and the latent image is converted to a visible image by causing toner to adhere thereto by magnetic brush development or other developing process.
This developing method makes use of a toner that converts the electrostatic latent image to a visible image and carrier particles comprising a magnetic material. The carrier particles provide a proper quantity of positive or negative electrostatic charges to the toner by triboelectric charging, and also carry the toner on the carrier particle surfaces by the electrostatic attraction force of the triboelectric charges.
The two-component type developer having such a toner and a carrier is coated on a developing sleeve provided with a magnet in its inside, in a given layer thickness by means of a developer layer thickness control member, and then transported to a developing zone formed between the photosensitive member described above and the developing sleeve.
A Given development bias voltage is applied across the photosensitive member and the developing sleeve. The toner is fed to the developing zone and transferred onto the photosensitive member.
There are various performances required in carriers. Particularly important performances are proper triboelectric charge-providing properties, breakdown strength against applied electric fields, impact resistance, wear resistance, anti-spent properties, developing performance and productivity.
For example, long-term use of the two-component type developer causes toner filming in which the toner that contributes no development (i.e, spent toner) melt-adheres to the surface of carrier particles, which consequently causes a deterioration of the two-component type developer and also causes a deterioration of image quality of developed images that is accompanied with it.
An excessively large true specific gravity commonly results in an increase in the load applied to the developer when the developer is made to have a given layer thickness on the developing sleeve by means of the developer layer thickness control member or when the developer is agitated in the developing assembly. Hence, (a) toner filming, (b) carrier break and (c) deterioration of toner tend to occur during the long-term use of the developer, so that the developer tends to deteriorate, accompanied with the deterioration of image quality of developed images.
An increase in particle size of the carrier also commonly results in an increase in the load applied to the developer and hence the above (a) to (c) are more likely to occur, so that the developer tends to deteriorate. It also brings about (d) a poor fine-line reproduction of the developed images, resulting in a poor developing performance.
Thus, the carriers that tend to cause the above (a) to (c) make it necessary to take troubles to periodically change developers, and are enconomically disadvantageous. It is necessary to decrease the load applied to the developer or improve impact resistance and anti-spent properties of carriers so that the above (a) to (c) can be prevented so as to make the lifetime of developers longer.
To cope with the problem on developing performance as noted in the above (d), it is necessary to make the particle size of carriers smaller.
To cope with the problems (a) to (d), a small particle size carrier comprising a binder resin and magnetic particles dispersed therein may be used, as exemplified by a magnetic material dispersed type small particle size carrier prepared by pulverization, as disclosed in Japanese Patent Application Laid-open No. 54-66134.
A magnetic material dispersed type small particle size carrier prepared by polymerization may also be used, as disclosed in Japanese Patent Application Laid-open No. 61-9659.
However, unless a large quantity of magnetic material is added to carrier particles, the above magnetic material dispersed type small particle size carriers have so small a saturated magnetization for their particle size that they have a problem of (e) adhesion of carrier to photosensitive members, which may occur during development. This makes it necessary to replenish the developer or provide in an image forming apparatus a mechanism for collecting adhered carriers. Thus, there cannot be drastic countermeasures for making the lifetime of developers longer.
In the case when a large quantity of magnetic material is added to the magnetic material dispersed type small particle size carriers, the quantity of the magnetic material increases with respect to the binder resin and hence the impact resistance becomes weak. This tends to cause falling-off of the magnetic material from the carrier particles when the developer is made to have a given layer thickness on the sleeve by means of the developer layer thickness control member. As a result, the developer tends to deteriorate. Thus, also in this case, these measures cannot be used as drastic countermeasures for making the lifetime of developers longer.
In the case when a large quantity of magnetic material is added to the magnetic material dispersed type small particle size carriers, resistance of the carrier decreases because of an increase in the quantity of a magnetic material having a low resistance. As a result, they tend to cause (f) faulty images because of a leak of the bias voltage applied during development.
To cope with such problems, a technique in which a magnetic powder is dispersed in a polyester resin is proposed, as disclosed in Japanese Patent Application Laid-open No. 59-157657. The polyester resin, however, has a problem. It commonly has so high a moisture absorption that its properties to provide charges to toner may greatly vary because of influences of temperature and humidity, when used as binders of carriers.
Use of a polyamide resin as a binder resin is disclosed in Japanese Patent Application Laid-open No. 2-22671. The polyamide resin has also a problem. It has relatively so large a surface energy that it can not bring about a satisfactory anti-spent properties, and also greatly tends to cause agglomeration of the carrier itself that it may bring about poor blending properties to toner, resulting in an unstable image density.
Meanwhile, in order to prevent carrier particles from filming on their surfaces, it has been proposed to coat carrier particle surfaces with a resin of various kinds. Such a method, however, is sought to be further improved.
For example, carrier particles coated with a fluorine resin such as a copolymer comprising ethylene tetrafluoride, where the resin has a low critical surface tension, may be filmed with toner with difficulty. The fluorine resin, however, has so poor film-forming properties that it is difficult for carrier core particles to be well uniformly covered therewith thereby making it hard to achieve a stable charge performance. The fluorine resin may also weakly adhere to core particles and can provide only a poor wear resistance. Moreover, because of a relation with the triboelectric series, the fluorine resin-coated carrier particles can not have a satisfactory charge performance in the case of negatively chargeable toners.
As for carrier particles coated with an acrylic resin such as a styrene/methacrylate copolymer, the resin has good film-forming properties and a strong adhesion to carrier core particles, which also has a superior wear resistance, and is used in combination with the above fluorine resin or alone. This acrylic resin, however, has so relatively high a critical surface tension that the carrier particles still tend to be filmed with toner, bringing about some problem in the lifetime of the developer.
In Japanese Patent Applications Laid-open No. 2-239255 and No. 2-239256, a polycarbonate resin is proposed as the carrier coating resin. When commonly available polycarbonate resins are used as carrier coating materials, the carrier may undergo changes in charge performance as a result of its long-term use, to often cause variations of image density and background stain at non-image areas. These can be caused, e.g., by an insufficient strength of adhesion to carrier core particles in the case of sole use of the polycarbonate resin, which may cause the resin to become separate from the carrier particle surfaces as a result of long-term repeated use, or by a high resistance inherent in the polycarbonate resin, which may cause attraction of external additives of toner to the surfaces of carrier particles, the former being retained on the latter as they are, when the carrier is brought into friction with the toner, resulting in a lowering of its charge-providing performance to the toner.
The problems may also be caused by an unsatisfactory compatibility that may be obtained when a different resin is used in combination in order to improve the adhesion of the polycarbonate resin to the core particles, which rather cause the filming with toner or falling-off of resin from the core particles, where it is difficult for the properties inherent in the polycarbonate resin to be well exhibited.
An improvement in film-forming properties necessarily makes the carrier have a high resistance, and causes charge-up of toner to bring about poor separation of the toner from the carrier.
Moreover, when development is carried out for a long time in the state the toner has thus become poorly separable from the carrier, the aforesaid filming of carrier with toner may be promoted to bring about undersirable results.
If a carrier has an excessively high resistance, the carrier may cause a decrease in image density and a deterioration of halftone reproduction at solid areas, or may adhere to a photosensitive member to cause scratches of the photosensitive member and also may adhere onto images.
Thus, it is very important not to damage film-forming properties of coating materials while controlling the resistance on carrier particle surfaces.